1. Field of the Invention
The present invention relates to a photosensor and a flame sensor including a filter device provided on a light incident face of a light receiving device, the filter device comprising an interference filter structure deposited on a colored glass filter.
2. Description of Related Art
Conventionally, there has been proposed a photosensor made by first preparing a colored glass filter by adding a color coupler (metal or metal compound) to glass and then assembling this colored glass filter with a light receiving device. This photosensor is configured such that light of a predetermined wavelength range is cut off by the colored glass filter and light of a desired wavelength range passing the colored glass filter is detected (see e.g. FIG. 1 of Japanese Patent Application “Kokai” No. 11-153483). For instance, for selectively detecting ultraviolet light contained in sunlight ranging from the ultraviolet range to the infrared range, a photosensor is constructed with a colored glass filter capable of shielding the visible range light and the infrared range light so that the ultraviolet light alone can reach the light receiving device to be detected.
One exemplary application of such photosensor as above is a sensor designed for UV-A (wavelength 315 nm to wavelength 400 nm), UV-B (wavelength 280 nm to wavelength 315 nm) or UV-C (wavelength 100 nm to wavelength 280 nm) as its detection target. And, for each such sensors, there is required wavelength selectivity for detecting only light of a certain wavelength included within each detection target wavelength range. Another exemplary application is an illuminometer designed for determining a light intensity of a light source employed in a photolithography process of a semiconductor exposing apparatus. When an exposure of a photoresist is to be effected using one of a line spectrum such as a g-spectrum (wavelength: 436 nm), h-spectrum (wavelength: 405 nm) and i-spectrum (wavelength: 365 nm) of a mercury lamp, this also requires selective determination of a quantity of light by a target spectrum.
However, in order to obtain better wavelength selectivity in the above-described photosensor, it is necessary to vary very sharply the intensity of light past the filter device at a predetermined wavelength (shielding wavelength) and also to have high sensitivity for the detection target wavelength range as high as 10,000 times or more of sensitivities for non-target wavelength ranges. The colored glass filter is prepared by adding impurity such as a metal or a metal compound to the glass so as to absorb/diffuse the light of a predetermined wavelength included in the incident light. However, as its light transmission characteristics showing the spectra of transmission lights slowly change relative to the wavelength, the light past the filter device still contains light of a wavelength which should be shielded by the filter device. Hence, the above-described sensitivity difference cannot be fully obtained.
In order to solve such problem relating to the wavelength selectivity, it is conceivable to improve the wavelength selectivity of the filter device through deposition thereon of an interference filter structure capable of shielding light of a predetermined wavelength range through utilization of an interference effect, the interference filter being comprised of a plurality of light transmitting layers deposited on the surface of the colored glass filter or on a light receiving face of a light receiving device.
However, since the interference filter structure shields the light of predetermined wavelength range through the light interference function, the wavelength range which can be shielded by the filter device is very narrow. This necessitates a plurality of interference filter structures having mutually differing shielding wavelength ranges to be deposited on the surface of the colored glass filter or on the light receiving face of a light receiving device. However, it has been found that with thick deposition of interference filter structures, there occurs a new problem of cracks therein. Further, the state-of-the art filters designed for selectively transmitting UV-B or UV-C alone suffer poor wavelength selectivity in permitting good transmission of light of a detection target wavelength while effectively shielding lights of non-target wavelengths. For instance, at the present state of the art, the light transmittance values for two wavelengths which are adjacent a target wavelength range and a non-target wavelength range, respectively, have a difference as small as about 1% even in the case of the interference filter. The difference is substantially non-existent in the case of a colored glass filter. For this reason, it has been impossible to detect, with high precision, UV-B or UV-C contained in the sunlight and having a very weak light intensity And, there is another problem that due to emission of line spectra from the mercury lamp other than the desired line spectrum, integrated sensitivities for the longer wavelength side of the light receiving device are inadvertently added to the output of the illuminometer.
In addition, substantially no colored glass filter is presently available which is capable of good transmission of the ultraviolet light. It is only known that a colored glass filter including a base material comprised of boron oxide, aluminum oxide, magnesium oxide, etc. has relatively good transmittance for ultraviolet. However, such colored glass filter having such ultraviolet transmission characteristics and having also effective shielding characteristics for visible range light is not comprised solely of such stable base material as SiO2 or Al2O3. Therefore, it is known that such colored glass filter suffers the problem of occurrence of undesirable change in its optical characteristics due to environmental influence such as heat or moisture. And, this influence will be even greater when the colored glass filter is employed under such environment of high temperature/high moisture. Especially, if a filter device using the colored glass filter is combined with a light receiving device to construct a photosensor expected to selectively detect light of a predetermined wavelength range alone as described above, such change with time in the optical characteristics of the colored glass filter (namely change in wavelength/intensity of the light incident on the light receiving device) will be a serious problem.
For example, in case the photosensor having the above-described construction is employed as a flame sensor of a type expected to provide selective detection of ultraviolet light, in view of the fact that the light intensity of the flame appearing in a wavelength range (wavelength from about 300 nm to about 280 nm) different from external disturbance light such as sunlight or room light from various lighting equipments, it is necessary to reliably shield such external disturbance light alone by means of a filter device having good wavelength selectivity. In this, as the flame sensor can be significantly affected by heat and/or moisture, with change, if any, in the optical characteristics of the colored glass filter, the light of the flame which should be incident on the light receiving device can be shielded inadvertently by the filter device, or conversely external disturbance light which should be shielded by the filter device can enter inadvertently the light receiving device. As a result, the result of flame light detection by such flame sensor (photosensor) will be unreliable.
The present invention has been made in view of the above-described problems. A primary object of this invention is to provide a photosensor and a flame sensor having a filter device capable of maintaining stable optical characteristics for an extended period of time.